Determination of the plasma effective charge from the soft X-ray spectrum and plasma conductivity at the L-2M stellarato
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IED PHYSICS
Determination of the Plasma Effective Charge from the Soft X-Ray Spectrum and Plasma Conductivity at the L-2M Stellarator after Boronization of the Vacuum Chamber A. I. Meshcheryakov and I. Yu. Vafin Prokhorov General Physics Institute, Russian Academy of Sciences, ul Vavilova 38, Moscow, 119991 Russia e-mail: [email protected], [email protected] Received April 6, 2015
Abstract—Boronization of the vacuum chamber wall results in a considerable change in the composition of the plasma generated in working pulses of the L-2M stellarator and, accordingly, in the plasma effective charge. The paper presents results of measurements of the plasma effective charge carried out by two methods in the ohmic heating mode: from the data on the plasma conductivity and from the soft X-ray spectrum of plasma emission. Comparison of the values of the plasma effective charge obtained by these two methods makes it possible to determine the conditions in which the two values are in good agreement. Under these conditions, the plasma effective charge can be correctly estimated from spectral measurements. DOI: 10.1134/S1063780X16070084
INTRODUCTION One of the parameters indicating successful plasma confinement in toroidal magnetic systems is the plasma effective charge Zeff,
Z eff
∑n Z = ∑n Z i
2 i
i
i
i
,
column is Te = 250–350 eV, and the central-chordaveraged plasma density is ne = (0.5–1.7) × 1019 m–3. The plasma effective charge in the quasi-stationary stage of the discharge was determined from the data on the conductivity of the plasma column and from spectral measurements.
(1)
i
where ni is the density of ions in the plasma column and Zi is the charge of the corresponding ion. Here, summation is performed over all charge states of all impurities existing in the plasma, i.e., n Z = ne . i i i Measurements of the plasma effective charge on the L-2M stellarator were already performed earlier [1]; however, optical measurements shows that the impurity composition changes significantly after boronization of the vacuum chamber [2]. This should result in a substantial change in the plasma effective charge. Therefore, it becomes necessary to measure the plasma effective charge under new conditions. This is the main aim of this work. The L-2M stellarator is a classical stellarator with a two-pole winding (l = 2). The parameters of the discharge and plasma vary within the following ranges: the loop voltage in the stationary stage of the discharge is UL = 2.1–2.8 V, the plasma current is Ip = 17–19 kA, the electron temperature in the center of the plasma
∑
COMPUTATION OF THE EFFECTIVE PLASMA CHARGE FROM THE MEASURED RESISTANCE OF THE PLASMA COLUMN In the mode with plasma heating by the induction current, the plasma effective charge was determined by comparing the computed resistance of the plasma column with it measured value Rexp = UL/Ip, where UL is the loop voltage and Ip is the plasma current. The plasma column resistivity was computed using the results obtained by Yu.N. Dnesterovskij and D.P. Kostomarov in [3]. In t
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